Enantioselective synthesis and enhanced circularly polarized luminescence of S-shaped double azahelicenes

Article abstract of DOI:10.1021/ja500841f

The enantioselective synthesis of azahelicenes and S-shaped double azahelicenes has been achieved via the Au-catalyzed sequential intramolecular hydroarylation of alkynes. The use of excess AgOTf toward a Au(I) complex is crucial for this transformation. Interestingly, the circularly polarized luminescence activity of the S-shaped double azahelicenes was significantly higher than that of the azahelicenes.

Full text of DOI:10.1021/ja500841f

Communication
pubs.acs.org/JACS
Enantioselective Synthesis and Enhanced Circularly Polarized
Luminescence of S‑Shaped Double Azahelicenes
Kyosuke Nakamura,† Seiichi Furumi,§,⊥ Masayuki Takeuchi,∥ Tetsuro Shibuya,† and Ken Tanaka*,†,‡
†Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and Technology, Koganei,
Tokyo 184-8588, Japan
‡ACT-C, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan
∥
§Applied Photonic Materials Group and Organic Materials Group, National Institute for Materials Science (NIMS), 1-2-1 Sengen,
Tsukuba, Ibaraki 305-0047, Japan
⊥PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
S
* Supporting Information
Scheme 1
ABSTRACT: The enantioselective synthesis of azaheli-
cenes and S-shaped double azahelicenes has been achieved
via the Au-catalyzed sequential intramolecular hydro-
arylation of alkynes. The use of excess AgOTf toward a
Au(I) complex is crucial for this transformation.
Interestingly, the circularly polarized luminescence activity
of the S-shaped double azahelicenes was significantly
higher than that of the azahelicenes.
elicenes, possessing fascinating π-conjugated helical
Hstructures, have long attracted much attention because of
their potential applications to functional organic materials.1
Therefore, research on the structure−chiroptical property
relationship2 and asymmetric synthesis of helicenes3 is highly
important. With respect to the structure−chiroptical property
chiral azahelicene E through axially chiral biaryl D by using a
single chiral transition-metal catalyst (Scheme 1).14 In this paper,
relationship, several studies on electronic and steric effects by the
introduction of substituents on helicenes have been reported
we disclose the Au-catalyzed enantioselective synthesis and
recently,2 while chiroptical properties of double helicenes have
enhanced circularly polarized luminescence of S-shaped double
not been well investigated to date.4,5 With respect to the
azahelicenes.
We first examined the reaction of diyne 1a15 with a cationic
asymmetric synthesis, several catalytic enantioselective syntheses
of helicenes have been developed by applying the transition-
Pd(II) or Au(I)/(R)-BINAP [2,2′-bis(diphenylphosphino)-1,1′-
metal-catalyzed aromatic ring construction.6−10 As the most
successful example, the transition-metal-catalyzed [2 + 2 + 2]
cycloaddition12 of alkynes has been applied to the catalytic
enantioselective synthesis of helicenes and moderate to high
enantioselectivities have been accomplished by using chiral
Rh(I),7 Ni(0),8a−c Co(I),8d and Ir(I)9 complexes. However, the
enantioselective synthesis of an azahelicene11 and the double
helicene3 has not been accomplished to date.
binaphthyl] complex.13 Although no reaction was observed using
the Pd catalyst, the use of the Au catalyst afforded biaryl 2a in
good yield (Table 1, entry 1). Interestingly, the use of excess
AgOTf toward the Au complex16 promoted the desired
sequential hydroarylation to give biaryl 2a and azahelicene 3a
in the same yields (entry 2). Elevating the reaction temperature
completed the sequential hydroarylation to give 3a in excellent
yield, while the ee value of 3a significantly decreased (entry 3).
Finally, increasing the amount of the Au catalyst afforded 3a in
excellent yield with a good ee value (entry 4).
Recently, the transition-metal-catalyzed intramolecular hydro-
arylation of alkynes has also been applied to the catalytic helicene
synthesis,12 while an enantioselective variant has not been
developed. However, our research group reported the
enantioselective intramolecular hydroarylation of alkynes to
produce axially chiral 4-aryl-2-quinolinones with high ee values
by using chiral Pd(II) or Au(I) catalysts (Scheme 1).13 In this
transformation, achiral alkyne A is efficiently converted to axially
chiral biaryl B through the formation of a pyridone ring. We
anticipated that the enantioselective sequential intramolecular
hydroarylation of achiral diyne C would proceed to give helically
The substituent effect of the enantioselective azahelicene
synthesis was examined, which revealed that a 2-methoxyphenyl
group at the alkyne terminus and two naphthyl groups are
necessary to obtain high product yields and/or high ee values
(Table 2). The reaction of phenyl-substituted diyne 1b15
proceeded to give aza[6]helicene 3b with a lower yield and ee
Received: January 24, 2014
Published: March 26, 2014
© 2014 American Chemical Society
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Communication
thermal stabilities of thus obtained azahelicenes 3a−e toward
racemization were examined, which revealed that all azahelicenes
retained their initial ee values after heating in (CH2Cl)2 at 80 °C
for 24 h, even in the case of aza[4]helicene 3e.
Table 1. Screening of Catalysts for Enantioselective
Sequential Intramolecular Hydroarylation of Diyne 1a to
a
Azahelicene 3a
Diyne 1f, possessing two naphthyl groups and a removable 4-
methoxybenzyl group on the nitrogen, was prepared starting
from known alkynoic acid 413 as shown in Scheme 2. The
formation of carboxylic acid chloride from 4 with 1-chloro-
N,N,2-trimethyl-1-propenylamine followed by amidation with
aminonaphthalene 5 furnished the corresponding amide 6. Acid
hydrolysis of the MOM protecting group furnished the
corresponding naphthol 7. Triflation followed by the Pd-
catalyzed Sonogashira cross-coupling with arylacetylene 8
afforded the desired diyne 1f. This diyne 1f was subjected to
the Au-catalyzed enantioselective sequential intramolecular
hydroarylation to give aza[6]helicene (−)-3f in high yield with
a good ee value. Removal of the 4-methoxybenzyl group on the
nitrogen followed by chlorination with phosphoryl chloride
afforded aza[6]helicene (−)-9f, possessing a pyridine unit,
without racemization.
The successful enantioselective synthesis of azahelicenes
prompted our investigation into the diastereo- and enantiose-
lective synthesis of S-shaped double azahelicenes via the Au-
catalyzed double sequential intramolecular hydroarylation of
achiral tetrayne 1g as shown in Scheme 3. Triflation of
commercially available 2-hydroxy-1-naphthaldehyde (10) fol-
lowed by the Pd-catalyzed Sonogashira cross-coupling with 8
furnished alkyne 11. The Corey−Fuchs reaction followed by
methoxycarbonylation and alkaline hydrolysis furnished alkynoic
acid 12. Etherification of 4-hydroxybenzonitrile (13) followed by
reduction furnished amine 14. The copper(I)/DMPAO [2-(2,6-
dimethylphenylamino)-2-oxoacetic acid]-catalyzed double C−N
bond formation,17 between 14 and 2,6-dibromonaphthalene,
afforded diamine 15. Condensation of thus obtained acid 12 and
diamine 15 afforded the desired tetrayne 1g. Although a high
catalyst loading was required, the Au-catalyzed enantioselective
double sequential intramolecular hydroarylation of 1g proceeded
at room temperature.18 Pleasingly, enantiopure S-shaped double
azahelicene (+)-3g19 and a mixture of racemic-3g and meso-3g
were separately isolated through silica gel preparative TLC. This
phenomenon can be explained by the fact that the excess
enantiopure (+)-3g (Rf = 0.45) traveled slower than meso/rac-3g
(Rf = 0.64) on the TLC plate due to the lower solubility of
enantiopure 3g than meso/rac-3g in the mobile phase (hexane/
EtOAc = 1:1).20 Indeed, enantiopure (−)-3g was obtained using
(R)-BINAP in place of (S)-BINAP. Removal of the 4-
alkoxybenzyl group on the nitrogen followed by chlorination
afforded S-shaped double azahelicene 9g,19 possessing two
pyridine units.
(R)-
2a
3a
b
b
AuCl(SMe2) AgOTf
BINAP
temp % yield
% yield
entry
1
(mol %)
(mol %) (mol %)
(°C)
(ee,
)
(ee,
)
20
20
30
30
45
10
10
10
15
rt
76
(56, +)
0
2
3
4
20
20
30
rt
42
(61, +)
42
(79, −)
80
(56, −)
96
(69, −)
80
rt
0
0
a
1a (0.050 mmol) and (CH2Cl)2 (2.0 mL) were used. Absolute
configurations of 2a and 3a were not determined. Isolated yield.
b
Table 2. Substituent Effect on Enantioselective Sequential
Intramolecular Hydroarylation of Diynes 1 to Azahelicenes 3
a
a
Reactions were conducted using AuCl(SMe2) (0.030 mmol), AgOTf
The photophysical properties of azahelicenes (3f and 9f) and
S-shaped double azahelicenes (3g and 9g) are summarized in
Table 3.21 Double azahelicenes showed red shifts of absorption
and emission maxima compared with azahelicenes. Double
azahelicenes showed higher quantum yields in CHCl3 solution
than azahelicenes, and that of 3g (ΦF = 0.190) was the highest
among four azahelicenes.22 The optical rotation values of double
azahelicenes 3g and 9g were smaller than those of single
azahelicenes 3f and 9f possibly due to the presence of two
pseudo-axially chiral methoxyphenyl groups. Finally, we
conducted circularly polarized luminescence (CPL) measure-
ments (differential emission of right circularly polarized light
versus left circularly polarized light in chiral molecular systems)
of four azahelicenes in chloroform. Unfortunately, CPL
(0.045 mmol), (R)-BINAP (0.015 mmol), 1a−e (0.10 mmol), and
(CH2Cl)2 (2.0 mL) at rt for 72 h. The cited yields are of the isolated
b
products. Absolute configurations of 3a−e were not determined.
A
reaction was conducted using AuCl(SMe2) (0.050 mmol), AgOTf
(0.10 mmol), (S)-BINAP (0.025 mmol), 1c (0.10 mmol), and
(CH2Cl)2 (2.0 mL) at rt for 7 days.
value than in the case of 3a (entry 1 vs 2). Furthermore, the
reaction of terminal diyne 1c15 was sluggish and the
corresponding aza[6]helicene 3c was obtained with a low yield
and ee value (entry 3). Diynes 1d,e,15 possessing dimethox-
yphenyl groups in place of naphthyl groups, were converted to
the corresponding aza[5] and [4]helicenes 3d,e in high yields,
while their ee values were low to moderate (entries 4 and 5). The
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Communication
Scheme 2
Scheme 3
a
Table 3. Photophysical Properties of Azahelicenes and S-Shaped Double Azahelicenes
e
UV-absorption
λmax/nm
fluorescence λmax/nm
(excitation wavelength/nm)
ϕF (excitation
wavelength/nm)
glum = 2(IL − IR)/(IL + IR)
d
compd
[α]25
(excitation wavelength/nm)
D
b
b
b
(−)-3f
(−)-8f
(+)-3g
(−)-9g
317, 381
467 (317)
0.051 (400)
1355
1273
377
<0.001 (375)
b
b
b
325
467 (317)
0.021 (380)
<0.001 (375)
c
c
c
284, 448
471, 492 (284)
0.190 (400)
0.028 0.002 (375)
c
c
c
260, 329, 445
454, 480 (329)
0.094 (400)
1086
0.011 0.001 (375)
a
b
c
d
e
Measured in CHCl3 at 25 °C. At 1.0 × 10−5 M. At 1.0 × 10−6 M. Values are calculated as 100% ee. The ee values of the compounds are as
follows: (−)-3f and (−)-9f, 74% ee; (+)-3g and (−)-9g, >99% ee.
intensities for azahelicenes 3f and 9f were below our measurable
limit (glum < 0.001). However, surprisingly, double azahelicenes
3g and 9g exhibited CPL activities, as the CPL spectra of (+) and
(−) helicenes are mirror images. The degree of CPL is given by
The use of excess AgOTf toward a Au(I) complex is crucial for
this transformation. Interestingly, the circularly polarized
luminescence activity of the S-shaped double azahelicenes was
significantly higher than that of the azahelicenes. Future work will
focus on the synthesis of novel helicenes, showing good
chiroptical properties, via the catalytic enantioselective aromatic
ring construction.
the luminescence dissymmetry ratio, which is defined as glum
=
2(IL − IR)/(IL + IR), where IL and IR are the luminescence
intensities of left and right circularly polarized light. The values
were glum = 0.028 at 492 nm for (+)-3g and glum = −0.011 at 454
nm for (−)-9g in chloroform; those values are comparable to
recently reported helically chiral small organic molecules
showing high CPL activities.23
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures and compound characterization data.
This material is available free of charge via the Internet at http://
pubs.acs.org.
In conclusion, we have achieved the enantioselective synthesis
of azahelicenes and S-shaped double azahelicenes via the Au-
catalyzed sequential intramolecular hydroarylation of alkynes.
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Communication
Rajca, S. J. Am. Chem. Soc. 2005, 127, 13806. (d) Carreno, M. C.;
Gonzalez-Lopez, M.; Urbano, A. Chem. Commun. 2005, 611. (e) Rajca,
́ ́
A.; Miyasaka, M.; Pink, M.; Wang, H.; Rajca, S. J. Am. Chem. Soc. 2004,
̃
AUTHOR INFORMATION
Corresponding Author
tanaka-k@cc.tuat.ac.jp
Notes
■
126, 15211. (f) Carreno, M. C.; García-Cerrada, S.; Urbano, A. Chem.
̃
Eur. J. 2003, 9, 4118.
(11) For the asymmetric (nonenantioselective) synthesis of
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported partly by ACT-C from JST (Japan) and
a Grant-in-Aid for Scientific Research (No. 20675002) from
MEXT (Japan). We thank the Nanotechnology Platform Project
of MEXT, Japan. We also thank Takasago Int. Co. for the gift of
DTBM-Segphos and Central Glass Co., Ltd. for the gift of triflic
anhydride.
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